1. Field of the Invention
The present invention is related to a laser optical system, particularly a laser optical system to preferably apply to a light recording apparatus for recording an information in form of letters or the like on certain recording medium by using a laser that oscillates in a multi-transverse mode of a phase synchronous semiconductor laser or a broad contact semiconductor laser.
2. Description of the Related Art
As seen in FIG. 3, a phase synchronous semiconductor laser in constructed as a multi-stripe type of laser diode array which is formed by dividing activation laser formed along a pn conjunction plane 26, and forming a multi-heterogeneous construction of a striped laser oscillation area 28 parallel arranged with several to several tens in one row. In the above stated construction, each laser oscillation area functions in a basic transverse mode and is connected to the adjacent laser oscillation area in an energy level. For that reason, a phase of an emitted laser beam from each laser oscillation area becomes synchronous, and oscillates in the transverse mode, so that a high output is obtained. A semiconductor laser of this type already exists in SDL-2410, SDL-2420 (Spectra Diode Labs Co., trademark) series.
Furthermore, it is known that there is phase difference in the laser beam emitted from the laser oscillation area in such a semiconductor laser, and when the laser beams emitted from each the oscillation area has differences of 180.degree., as shown in FIG. 4, two parallel lobes are formed in a direction along the pn conjunction plane in the far field pattern. Consequently, though the laser beam which forms two lobes is applied for recording on the recording medium, since the laser beam does not focus in one spot of a recording medium, a high resolution optical system can not be realized. Especially, if the laser beam is used for recording lettered information by dots on a microfilm, where a resolution power of about 3360/7.2 mm is required, the above stated two lobes cause problems since it is necessary to record each dot of the information. Further, the broad contact semiconductor laser, solid laser, and a gas laser have the same problems as the phase synchronous semiconductor lasers because of oscillating in the multi-transverse mode.
For this reason, the conventional phase synchronous semiconductor lasers are provided with a concavo-convexity phase correction plate which is arranged adjacent to nonreflective coating surface for producting an optical path difference (Appl. Phys. Lett., Vol. 50, No. 13, 30 Mar. 1987, page 783 FIG. 1), or a microscope objective is disposed at an emission side of the phase synchronous semiconductor laser, a phase correction plate similar to the above stated phase correction plate is disposed in a position of image formation formed by the objective on the coating surface of the semiconductor laser (IEEE JOURNAL OF QUANTUM ELECTRONICS. VOL. QE-22 No. 6, JUNE 1986, page 750, FIG. 3), so that a phase correction is achieved to obtain a Gaussian distribution.
Further, there are techniques related to the present invention, specifically applying techniques for cutting one of the two lobes by the optical system, which have been published in Appl. Phys. Lett. 41 (12), 15 Dec. 1982, page 28, FIG. 3 and in Japanese Patent Application Laid Open No. 62-98320.
However, in a case where a phase correction plate is arranged adjacent to the coating surface of the semiconductor laser, the concavo-convexities of the phase correction plate have to correspond to the stripe width which is several .mu.m degrees of the phase synchronous semiconductor laser, and manufacturing of the phase correction plate of the required preciseness is difficult. In the case of using an objective, a magnified image of the nonreflective coating surface is formed by the objective and the manufacture becomes simple in comparison with the above case. However, because it is necessary to arrange the phase correction plate at the point of image formation by the objective, adjustment tends to be troublesome. If an attempt is made at simplifying the manufacture further by magnifying the magnified image of the coating surface, the optical system becomes large size, and it becomes necessary to use a large size lens in the objective because the magnification ratio is determined by the ratio of the focal length of the lens used as the objective. Other types of lasers, like the broad contact lasers, have the same problems.